The present invention generally relates to a system and method for controlling slurry used for wire saw ingot slicing. In particular, the invention relates to a slurry handling system and method for controlling temperature and flow rate of the slurry and for preheating the ingot and web.
Semiconductor wafers are generally prepared from a single monocrystalline semiconductor ingot, such as a silicon ingot having a cylindrical shape. The ingot is sliced in a direction normal to its longitudinal axis to produce as many as several hundred thin, disk-shaped wafers. The slicing operation may be accomplished by means of a single or multiple wire saw, wherein the ingot is contacted with a reciprocating wire while a liquid slurry containing abrasive grains is supplied to a contact area between the ingot and the wire. As the abrasive particles in the slurry are rubbed by the wire against the ingot, silicon crystal is removed and the ingot is gradually sliced. The wire saw provides a gentle mechanical method for slicing which makes it ideal for cutting silicon crystal, which is brittle and could be damaged by other types of saws (e.g., conventional internal diameter saws). After slicing, each wafer is subjected to a number of processing operations to reduce the thickness, remove damage caused by the slicing operation, and create a flat and highly reflective surface suitable for fabrication of integrated circuit devices.
Wire saws generally have two, three or four rollers which are rotatably mounted on a frame, each roller having guide grooves for receiving segments of wire. Multiple parallel lengths of the wire extend between two of the rollers to form a wire web for slicing the ingot into multiple wafers. The space between adjacent wires in the web generally corresponds to the thickness of one wafer before processing. The apparatus includes an ingot support that may mount one or more silicon ingots and is adjustable to accurately align an orientation of the crystalline structure of the ingot relative to a cutting plane. The support is moveable in translation to bring the ingot into contact with the wire web.
Slurry is transported from a nearby slurry container to the wire by a pump, tubing, and at least one nozzle which dispenses slurry onto the wire web. A portion of the slurry then moves with the wire into a contact area between the wire and the ingot where the silicon crystal is cut. Typically, there are two nozzles positioned on opposite sides of the ingot holder so that slurry is dispensed onto the web on both sides of the ingot, thus facilitating delivery of slurry to the cutting region for either direction of travel of the reciprocating wire. Each nozzle is positioned above the wire web at close spacing and configured to dispense slurry in a generally thin, linear and homogeneous distribution pattern, forming a curtain or sheet of slurry. The slurry curtain extends across a full width of the wire web so that slurry is delivered to every reach of wire and every slice in the ingot.
A substantial concern when slicing semiconductor ingots is maintaining flatness of the wafers that are cut by the wire saw. One key to avoiding thickness variation and warp on wafer surfaces is controlling build up of frictional heat at the contact area, or cutting region. Accordingly, the liquid slurry is actively cooled prior to dispensing on the wire web so that it may remove heat as it passes through the cutting region. For cooling the slurry, a heat exchanger is typically located between the slurry-collection container and the slurry delivery nozzle.
A limitation to the process of slicing semiconductor ingots is that it requires a substantial amount of time and can become a hindrance to the efficient production of wafers. It is desirable to slice the ingots as quickly as possible to improve throughput and reduce costs, yet there have been difficulties implementing a more rapid wire sawing process. The speed of the cutting wire cannot be substantially increased because that would elevate temperature at the cutting region to the detriment of the flatness of the wafers and their surface finishing. In addition, high wire-speeds relative to the ingot increase the possibility of wire breakage that is detrimental to the process outcome.
The use of multi-wire saw process for slicing large work pieces (200 mm and greater in diameter) has emerged as one technology of choice for meeting the requirements of both the semiconductor as well as the photovoltaic industries. Primary growth drivers of the technology have been its abilities not only to process multiple slices simultaneously but also to be able to slice small thicknesses with minimal kerf loss. The slicing technology in its currently practiced industrial form is based on employing the cutting action of free floating slurry particles in a process fluid (mineral-oil/glycol). This slurry is introduced on a web of wires which carry it into the ingot cutting zone by a periodic reciprocating motion of the wires. The ingot to be sliced is pressed against this reciprocating web of wires and is progressively sliced by the cutting action of the slurry particles by a rolling, indenting, cutting, scratching mechanism.
Unfortunately, the slicing technology in its industrially practiced form has been arrived at mostly through empirical means, and little is reported/understood about the fundamental mechanisms that lead to the surface features observed on the as-cut wafers. Therefore, with the requirements for better surface finish getting tighter by the day, it is becoming increasingly difficult to produce wafers with ever decreasing warp without controlling the process based on more fundamental understanding. Hence, producing as-cut wafers with reduced warp while lowering the total cycle time is critical to minimize the cost of ownership of the wire saw process, in addition to producing wafers with higher surface quality.
FIG. 1 illustrates one related art system such as may be used with a wire saw and process for slicing multiple semiconductor ingots as disclosed in U.S. Pat. No. 6,941,940, the entire disclosure of which is incorporated herein by reference. A slurry cooling system 102 receives recycled slurry from a slurry collection system 104. The recycled slurry is cooled by system 102 and supplied via a valve 106 to a wire web port 108 of a wire saw process 110 for slicing ingots. The recycled slurry cooled by system 102 is also supplied via a valve 112 to an ingot holder port 114 of a wire saw process 110 for slicing ingots. Slurry supplied to the wire web port is applied to the wire web during the wire saw cutting of an ingot and slurry supplied to the ingot holder port is applied to the ingot holder during the wire saw cutting of the ingot. Thereafter, the applied slurry absorbs heat during wire saw cutting and the heated slurry is collected by the slurry collection system 104, which provides the heated slurry to the slurry cooling system 102 for cooling and recycling.
FIG. 2 is an exemplary illustration of a simulated two-dimensional wafer shape with respect to a best-fit reference plane from the related art process of FIG. 1 for a wafer cut by saw wire from the end of an ingot, where warp tends to be greater. The darker area centered about y=150 indicates a substantially flat area with respect to a “best-fit” reference plane. The darker areas at the edges about y=0 and y=300 indicate an area which is about 15 μm below the “best-fit” reference plane. FIG. 3 is an exemplary illustration of a simulated one-dimensional wafer shape profile with respect to a best-fit reference plane from the related art process of FIG. 1 for a wafer cut by saw wire from the end of an ingot, where warp tends to be greater. The vertical axis indicates a range of −20 μm below the “best-fit” reference plane to 10 μm above the “best-fit” reference plane.